Field of the Invention
This invention concerns nitrite-containing, corrosion-inhibiting greases and their preparation. More particularly, this invention concerns grease compositions incorporating combinations of inorganic nitrites with N-acyl sarcosines as their corrosion-inhibiting entity.
The use of corrosion-inhibiting compositions in oils and greases to minimize corrosion in moving parts such as bearings is well known in the lubricating art. In the absence of these compositions, metal parts, particularly those fabricated from the ferrous metals and their alloys, rapidly become oxidized in an environment of heat, moisture and free acidity. Once oxidized, these parts function unsatisfactorily, and prematurely require maintenance and/or replacement.
Corrosion inhibitors can be of different types or classes. Some are oil-soluble while others are oil-insoluble. Illustrative of the former type of inhibitors are the metal sulfonates and naphthenates, while the latter are exemplified by inorganic nitrite and borate salts. Generally, the oil-insoluble (water soluble) type of corrosion-inhibitors are preferred for static or quiescent operational conditions while the oil-soluble inhibitors have been favored in situations where the grease film is being constantly sheared and displaced (as in bearings). However, for applications where the greases are challenged by an environment of elevated temperatures and high humidity, neither the oil-soluble salts or the metal nitrates alone have been entirely satisfactory and novel and more effective combinations of inhibitors are sought. This is particularly the case in lithium soap-based greases and certain sodium soap-based greases where sodium nitrite alone in moderate amounts does not provide the desired degree of anti-corrosive protection. Increasing the quantity of nitrite is undesirable since in many instances it adversely affects the physical characteristics and stability of the grease. In view of this shortcoming much development work has been done to find materials which, when combined with the nitrites, will synergize or potentiate their corrosion inhibition.